Nesota



Oct 29,1929. D. B. HARRIS ET AL 1,733,194

TRANSMISSION SYSTEM FOR SOUND CARRYING- CURRENTS Filed July 31, 1926 Patented Get. 29, 1929 UNITED STATES PATENT OFFICE DONALD B. HARRIS, OF MINNEAPOLIS, AND ORVILLE W. KNAUSS, OF ST. PAUL, MIN- NESO'IA; SAID KNAUSS AESIGNOR F ONE-TENTH TO SAID HARRIS TRANSMISSION SYSTEM FOR Application filed July 31,

This invention relates to electrical telephone systems, and especially to such systems for use in amplifying sound carrying cur rents transmitted from a central station to a distantly located station through the medium of the ordinary conductors employed by a telephone or telegraph system.

Heretofore, a considerable amount of experimental and research work has been carried on at the instance of the telephone companies with a View to developing a system whereby musical programs, lectures, sermons, etc, could be transmitted to distantly located points through conductors in the conventional telephone or telegraph cables. Such a system would enable the telephone or telegraph companies to control the transmission of programs and to charge their subscribers for the service rendered. In these experiments the distantly located stations have been provided with specially constructed radio loud speakers connected directly to the conductors in the telephone cables. This arrangement necessitated the transmission of a relatively large amount of power in order to operate the loud speaker to capacity. Consequently, it was found necessary to impress a proportionally high voltage sound carrying current upon the conductors in the cable. The eflect of this high voltage was to cause a capacitative induction of energy into other conductors in the cable employed for the usual telephone purposes, and interference with telephone conversations resulted, rendering these experiments unsuccessful.

It is the main object of this invention to provide a highly efiicient electrical system, capable of transmitting electrical or audio frequency impulses from a central station to a distantly located station over conductors, such as the conductors in a conventional telephone cable, and capable of receiving and rendering such impulses audible through the medium of a loud speaking telephone at the distantly located station.

It is a further object to provide means in such a system for obviating the dihiculties en countered in the experimental work carried on previous to this time by employing novel amplifying apparatus at the distantly located SOUND-CARRYING CURRENTS 1926. Serial N0. 126,224.

stations, so connected to the conductors in the telephone cable that the loud speaker can be operated to capacity without causing interference with other conductors within the cable.

More particularly it is an object to provide an electrical system whereby musical or audio frequencies may be transmitted through conductors in a conventional telephone cable with a minimum energy content incapable of causing cross talk or interference with the proper functioning of other circuits in the same cable and whereby the energy of such currents can be augmented at the distant point to an extent sutiicient to produce sound with maximum energy content.

Another object is to provide in a system including a central station and a plurality of distantly located stations, amplifying sets located at the distantly located stations and connected to the central station by means of conductors in conventional telephone cables. and means for supplying from remote sources all electromotive forces necessary to maintain the several elements of the various amplifyin g sets at operative potentials, thereby avoiding the necessity for batteries or the equivalent thereof at the distantly located stations.

Preferably said sound carrying current and all electromotive forces supplied to the several distantly located stations with theexception of the current for maintaining the filament elements of the amplifying sets at incandescence are transmitted through said telephone conductors in combination with ground. In this system provision is made to prevent any flow of current resulting from said electromotive forces except when one or more of said sets are in operation.

In this system we preferably supply current to the filament in each amplifying set from the household source of alternating current. In this connection it is a further object to provide means for eliminating all alternating current hum in the amplifying and reproducing of the electrical or audio-frequency impulses at the several distantly located receiving stations.

These and other objects of the invention will be apparent from the following description made in connection with the accompanying drawing, wherein like characters refer to similar parts throughout the several views, and in which:

Fig. 1 is a schematic or electrical diagram of the preferred embodiment of the invention; and

Fig. 2 is a detailed view showing a fragment of a telephone or telegraph cable, spare conductors of which are utilized in our system.

The portion of the drawing at the left side of the vertical dotted line 0 represents apparatus installed at a central station, while the portion of the drawing at the right side of the line cc represents apparatus installed at a distantly located receiving station, such as the home of a telephone subscriber.

In the description of our electrical sys tem which follows, various proportions, ratios, values of elements, capacities and im pedances are enumerated. The system may, of course, be utilized with elements of different value, capacities, impedances, etc, and we have successfully demonstrated the success of the apparatus and system with values other than those given. The capacities and ratios stated are merely for the purpose of example, and it is to be clearly understood that we do not in any way limit the scope of the invention to the values, ratios and capacities enuu'ierated.

At the central s ation is located a radio receiving set or telephone repeater connected ing 3 is wound to possess an impedance preferabl y of one-fifth the impedance of element 2. in experimental worl: an impedance of approximately 2000 ohms at 1000 cycles has been found satisfactory for element 3. Together windings 2 and 3 constitute a step down transformer with an approximate ratio of one to five. This transforn er possesses comparatively straight line frequency characteristics, such that the majority of frequencies in the audible range are equally transmitted.

A pair of conductors in a conventional telephone or telegraph cable are indicated by the letters X and Y, connecting the apparatus in the central station with the apparatus in the distantly located station. A C-battery which may be approximately four and one half volts has its negative terminal connected to one end of the secondary winding 3, and

meagerc its positive terminal connected to the conductor Y, while a B-battery preferably of about 90 volts has its negative terminal connectcd to conductor Y and its positive terminal connected to ground.

Element 0 is an autotransformer having two distinct electrically and electro-magnetically continuous windings a and 3). Conductor X is connected to transformer 6 at the junction between windings a and b. ll inding i) is so wound as to possess an impedance approximately equaling the impedance of element 3. Element 0 forms a stepup transformer with an approximate ratio of preferably one to five. It also possesses straight line frequency characteristics similar to the transformer 2-3. A l-n'iicrofarad condenser T is connected to winding of autotransformer 6 and to conductor Y preventing any short circuit from the C-battery i and a second one-microfarad condenser 8 is connected to condenser 7, conductor Y and grounded.

Numerals 9, 10 and 12 indicate the grid, plate and filament of a thermionic radio aniplifying tube, disposed at the distant station. The impedance between grid 9 and filament 12 is approximately equal to that of winding a of autotransformer 0 and the grid Sis connected to the free terminal of said winding a. Element 11 is a potentiometer having a movable contact 11 and a fixed resistance coil 11". The terminals of resistance coil 11 are connected to the terminals of filament 1 and movable contact 11 is connected to conductor Y.

A step-down transformer of the bell ringing type is employed comprising the windings la and 15, winding 15 being nected at the distant station with the ordinary household source of electricity, and the element 14-. being so wound to obtain approxi mately 10 volts. I

A protective filament resistance element 13, is interposed between one terminal of element 14 and one terminal of element 11, adapted to keep the current flow through filament 12, at or less than .25 amperes. A rheostat 17, preferably of about ohms resistance is interposed between the other terminal of element 14, and the other terminal of potentiometer 11. A conventional radio loud speaker 10 with an impedance approximately equal to the plate to filament impedance of the ther mionic tube 9, 10, 12 is mounted at the distant station connected to plate 10 and ground.

0 ZMTCLZZIOW Sound carrying current potentials received from the radio station repeater or other sources are impressed on element 1 causing current to flow through the winding 2 of transformer 23. This current is tram-i ferred by electro-magnetic induction to winding 3 setting up a potential difierencc across winding 3 which is approximately one-fifth as great as the potential difference across ele ment 2. A path for this current is set up through winding 3, conductor X, part (b) of auto transformer 6, condenser 7, conductor Y, C-battery 4, back to winding 3, thus causing current to flow through part (b) of element 6. Said flow of current through part (b) of auto transformer 6 induces a sound carrying current of equal frequency in part (a) of transformer 6 and sets up a potential across the entire length of said transformer. This potential is approximately five times as great as the potential set up across part (b) of auto-transformer 6, and is approximately equal to, or greater than the original potential impressed on winding 2 of the first men tioned transformer. This potential is impressed on grid 9 and on the filament 12, through condenser 7 and element 11, thus causing the grid 9 to function in the conventional manner. Due to the functioning of grid 9 a second soundcarrying current potential similar in frequency but greater in amplitude and power is set up between plate 10 and filament 12, causing a current to flow from element 10 through the loud speaker 16 to the ground side of condenser 8, through condenser 8, through element 11, to the filament 12, and throrgh the tube back to plate 10, thus operating the loud speaker 16.

The grid of the tube is maintained at its operative potential in respect to filament 12 by means of a potential impressed on the circuit by C-battery 4. The path of this potential is from the negative pole of battery 4 through winding 3, through conductor K, through part (a) of auto transformer 6, to plate 9, and from the positive pole of C-battery 4, through conductor Y, through element 11, to filament 12. Plate current and potential for operation of plate 10 are supplied by means of a potential caused by B-battery 5. The path of this potential is from the negative pole of battery 5, through conductor Y, through element 11, to the filament l2, and from the positive pole of B-battery 5 through ground, to the ground on loud speaker 16, through element 16, to the plate 10 of the tube.

Current for maintaining the filament 12 at incandescence is derived from alternating current house lighting circuits. The path of this current is through winding 15, inducing a current in winding 14, similar in frequency but less in amplitude, through rheostat 17, to filament 12, and through resistance coil 13 to filament 12.

Due to the fact that the current used to light the filament is alternating current, the conventional method of connecting the grid return direct to one side of the filament would cause potentials to be set up between filament 12, and grid 9, which would be alternating in character, and which would cause the grid to function in its conventional manner, thereby producing similar alternating currents through the loud speaker 16 and causing an interfering hum. This contingency is avoided by shunting filament 12 with potentiometer 11 preferably having a resistance of 200 ohms. The grid return wire as well as the plate return wire (conductor Y) is connected to mow able contact 11* of the potentiometer. The contact is then set in a position such that alternating current potentials impressed upon the movable contact from the left side of resistance coil 11 shall be equal to and opposite in direction to alternating currents impressed upon the movable contact from the right side of resistance 11. The two alternating current potentials mentioned will thus neutralize one another and no effective alternating current potential in respect to filament 12 will be impressed on the grid 9. The alternating current hum is thus avoided.

Condenser 8 constitutes a bypass for sound carrying current potentials set up between the groiuid side of loud speaker 16 and the filament 12. If element 8 were not provided these sound carrying current potentials would normally be transmitted through ground, through B-battery 5, and through conductor Y, back to the filament 12, thus setting up sound carrying current differences of potential between conductor Y, and ground and causing a transfer of energy from conductor Y to other unconnected conductors in the same cable. Condenser 8 provides a path whereby sound carrying current differences of potential set up between the ground side of the loud speaker to the filament will be conducted direct from the loud speaker to the filament 12 without passing through con ductor Y.

It will be seen that the sound carrying current impulses received at the central station are impressed on element 1, causing current to flow through the winding 3, which current sets up a difference of potential approximately one-fifth as great as that set up by the last audion tube in the central station, due to the step down efiect of transformer 2-3. Thus, if the potential set up by the tube is 8 to 10 volts the potential impressed upon the cable pair will be two volts or less, which, tests have shown is insufficient to cause interference with the other pairs or conductors Within the same cable. Transmission of the sound carrying current impulses, therefore, over the conventional conductors within the telephone cable cannot interfere with the telephone'conversations transmitted through the cable.

This improved system obviates the necessity for the use of any batteries at the distant station. The electromotive forces are supplied to maintain the several elements of the amplifying set at the distant station, from the central station, through the conductors in the telephone cable and ground, and in combination with the source of household electricity at the distantly located station transformed and connected to the filament 12.

The above described system is readily adaptable for use by the telephone companies, since the cost for-installation of the apparatus at the various distantly located stations is relatively low, and since the servicing charges on such stations will merely amount to replacing or repairing the tubes in the several amplifying sets. The many conductors necessary to transmit the sound carrying currents and other electromotive forces necessary are pres ent in the pairs disposed within the conventional telephone cables. Batteries to supply the several grid and plate circuits in the various distantly located stations may be maintained at a central station, While the alternating current for the filament circuits may be supplied from the electric light circuits connected to the homes of telephone subscribers.

A high degree of eiliciency is obtained in the amplifying system, due to the relative equality of the windings of transformers (23) and 6.

All alternating current hum from the filament circuits is obviated by the employment of the potentiometer 11 in its specific comhination With the other elements of the apparatus installed at the distantly located station.

Although the circuits of this system include connections to ground there can be no flow of current or loss of electromotive forces exccpt at such times when the distantly located sets are in operation.

This invention has been thoroughly tested in the embodiment illustrated and. described and has been found highly successful for the purposes enumerated.

It will, of course, be understood that various changes may be made in the form, de tails, proportions and arrangement of parts Without departing from the scope of the invention.

What is claimed is:

1. A transmission system for sound carrying current comprising a source of sound carrying current, a distantly located thermionic amplifier having grid, filament and plate, a pair of conductors adapted to supply said sound carrying current to said amplifier and adapted to supply electromotive forces to said ampl ifier, whereby said grid and plate Will be maintained at operative potentials in respect to said filament, and a transformer system connected to said pair of conductors, adapted to keep potentials across said conductors due to said sound carrying current at a lOW value, and adapted to increase said potentials at said amplifier to a value adequate to operate said amplifier to capacity.

2. A transmission system for sound carrying current comprising a source of sound carrying current, a distantly located thermionic amplifier having grid, filament and plate, a pair of conductors adapted to supply said sound carrying current to said amplifier, and a transformer system adapted to keep potentials across said conductors due to said sound carrying currents at a lon value, adapted to increase said potentials at said amplifier to a value adequate to operate said amplifier to capacity, and adapted to permit transmission of electromotive forces to said amplifier whereby said grid and plate of said amplifier will be maintained at operative potentials in respect to said filament.

3. A transmission system for sound carrying current comprising a source of sound carrying current, a distantly located thermionic tube having a plate element, a grid element and a filament element, a source of electrical energy having a positive pole and a negative pole, a conductor connected to said grid element and to the negative pole of said source of electrical energy, a conductor con nected to said filament element, connected to the positive pole of said source of electrical energ and connected to the negative pole of a second source of electrical energy, the positive pole of said second source of electrical energy being connected to ground or earth, a connection between said plate element and ground, and a co-operating transformer system connected to said conductors whereby potentials across said conductors due to said sound carrying current are reduced to a low value, and whereby said potentials at said amplifier are increased to a value adequate to operate said amplifier to capacity.

4 In combination with an electrical communication system, including a pair of conductors in a conventional telephone cable, a source of sound carrying electrical energy connected to said conductors to set up thereacross a sullicient-ly low potential to prevent transfer of energy from said conductors to other conductors in said cable, a thermionic amplifier having a grid, plate and filament, said amplifier being; connected to said conductors at a distant point, a step-up transformer adjacent said amplifier and connected thereto and to said conductors, adapted to increase potentials impressed fromsaid conductors to a value adequate to operate said amplifier to capacity, and also adapted to perunit the transmission of electromotive forces from said conductors to said amplifier Whereby said plate and grid Will be maintained at operative potentials in respect to said filament.

5. In a system including a central station, a distantly located station and a conventional communication cable extending between said stations and having a plurality of conductor pairs therein. a source of sound carrying current at said central station, means at said central station for impressing upon a pair of said conductors sound carrying currents from said source at sufiiciently low potential to prevent transfer of energy from said pair to other conductors within said cable, an amplifying set at said distantly located station, a loud speaking telephone connected to said set, and means at said distantly located station interposed between said pair of conductors and said amplifying set for increasing the potential of said sound carrying current to a value suflicient to operate said amplifying set to capacity.

6. In a system including a central station, a distantly located station and a cable extending between said stations and having a plurality of conductor pairs therein, a source of sound carrying current at said central station, means for impressing upon a pair of said conductors sound carrying currents from said source at sufiiciently low potential to prevent transfer of energy from said pair to other conductors within said cable, an amplifying set at said distantly located station, a loud speaking telephone connected to said set, an autotransformer at said distantly located station, the ends of which are connected to said amplifying set and to one of said conductors and including an intermediate connection to the other of said conductors adapted to increase the potential of said sound carrying current to a value adequate to operate said amplifying set to capacity.

In testimony whereof we afiix our signatures.

DONALD B. HARRIS. ORVILLE W. KNAUSS. 

